Log Message:
Update to version 0.3.1.  The main point is to fix gnutls build failures.

- Support constant size bit strings, as in 'BIT STRING (SIZE(42))'.
  Reported by Cyril Holweck <cyril.holweck@q-free.com>.
- Add two more APIs required by GnuTLS.
- New public APIs:
  asn1_find_node function
  asn1_copy_node